1. Technical Field
The present invention relates to emulsions for preparing a surface for a powder coating. More specifically, the present invention relates to a conductive sol-gel emulsion.
2. Background Art
Coating compositions have long been used to provide the surface of articles with certain desired physical characteristics such as color, gloss and durability. In the past, most coating compositions elied upon a liquid carrier, which evaporated after the composition was applied. Recently, there has been a growing trend toward reducing the volatile organic compounds (VOCs) of coating compositions. One way of achieving this objective has been through the use of dry, solventless systems such as powder coating systems.
In typical powder coating systems, an electrostatic charge is often applied between the application device (e.g., the spray gun) and the item to be painted. This results in attraction and adherence of the powder coating to the desired substrate.
After being applied, the powder coating is cured. Typically, this is achieved by heating the coated substrate to an elevated temperature (e.g., a temperature between 50° C. and 400° C.). During the curing process, the powder particles melt and spread, while the components of the powder coating crosslink. In addition to not emitting any VOCs into the environment during the application or curing processes, such a powder coating system is extremely efficient since there is essentially no waste (i.e., application yield is approximately 100 percent).
Powder coating systems are well known to those skilled in the art. In the coating industry, the term “powder” includes not only materials with a relatively small particle size, but also those with larger sizes, such as granules. Typically, however, the average particle size of most power coating systems are in the range from about 0.1 to about 500 microns; and more commonly, from about 0.5 to about 100 microns.
Powder coating is a well-known method for the coating of objects in which one starts with a powder coating material which is electrically charged and sprayed against the surfaces of an object. The material is finally adhered and converted to a solid state by heating to its melting temperature. Since the powder consists of a plastic, which is cured by heating, it must be heated to a comparatively high temperature, about 200° C.
The coating method can be performed on objects having good heat resistance and a conductive surface. However, where the surface of an object is non-conductive, implying that the object cannot be grounded or supplied with a charge of an opposite polarity to the charge of the powder, difficulties arise with enabling the powder to adhere to the surface of the object during the time between spraying and heating the powder to the melting temperature.
The difficulty of obtaining a polarity difference between the powder and the object, when non-conductive surfaces are involved, has been addressed by either varnishing the object with a conductive varnish, or subjecting the object to water so as to form a conductive moisture layer on the surface. The methods are, however, of limited use because of numerous disadvantages. The disadvantages include the additional operation and material required for varnishing and inferior adhesion as compared to powder coating on a clean surface. Additionally, when utilizing such methods, discoloration can occur with clear varnishes. Further, the addition of water can impair the adhesion of the powder coating and can damage the object by confining the added water beneath the coating.
A further method of enabling the powder to adhere to the surface of a non-conductive object is disclosed in German Patent No. 3,211,282. The '282 patent teaches heating glass objects having good heat resistance to a temperature of 400-900° C. This causes the powder granules that impact the object to melt and stick to the surface. One problem with the method is that objects that are heat sensitive can deform when exposed to high temperatures and cannot be treated at the high temperature required by this method. Thus, the method of the Albers patent cannot be applied to heat sensitive objects such as wood or plastic. There is therefore no method available for powder coating heat sensitive object.
It would therefore, be useful to develop a coating, which overcomes the above problems with regard to wood and plastic while still utilizing the beneficial powder coating technology.